Technical Field
The present invention relates to shovels configured to have a swing mechanism driven by a hydraulic motor, and methods of driving a shovel.
Description of Related Art
A hydraulic motor configured to drive the swing mechanism of a shovel is driven with high-pressure hydraulic oil supplied from a hydraulic pump through a motor drive hydraulic circuit. The motor drive hydraulic circuit includes a pair of main conduits, namely, a conduit in which hydraulic oil supplied to the hydraulic motor flows and a conduit in which hydraulic oil discharged from the hydraulic motor flows. When one of the main conduits serves as a supply conduit, the other of the main conduits serves as a discharge conduit. To reverse the rotation direction of the hydraulic motor, the supply conduit and the discharge conduit are switched.
To stop the swinging of the rotating structure of the shovel, both of the main conduits of the motor drive hydraulic circuit are closed to stop the driving of the hydraulic motor. The rotating structure of the shovel, however, has a large inertia weight and cannot stop instantaneously. Therefore, even when the supply conduit is closed, the hydraulic motor tries to keep rotating because of the inertial force of the rotating structure.
With this, hydraulic oil discharged from the hydraulic motor flows into the closed discharge conduit to sharply increase the hydraulic pressure inside the discharge conduit. This increase in the hydraulic pressure inside the discharge conduit brakes the hydraulic motor, but an excessive increase in the hydraulic pressure may damage the discharge conduit. Therefore, a relief valve is provided in the discharge conduit to prevent the hydraulic pressure inside the discharge conduit from exceeding a predetermined pressure (a relief pressure), thereby preventing damage to the discharge conduit due to high pressure.
The hydraulic pressure of a discharge conduit is returned to a supply conduit through a variable relief valve according to a related-art motor drive hydraulic circuit, while hydraulic oil in the discharge conduit may be returned to a hydraulic oil tank through a relief valve.